Population studies on the infective stage of some nematode parasites of sheep

Abstract

Observations have been made on the distribution of faecal deposits on a number of small fields of varying size stocked with varying numbers of sheep. These studies were undertaken to examine certain hypotheses proposed in the construction of mathematical models for the distribution of strongyloid nematode infective larvae in pastures grazed by sheep.The negative binomial distribution was found to provide a reasonable empirical description of the distribution of faeces, although there was evidence of departure from the negative binomial on the largest field.Although, tentatively, the distributions were additive with respect to the number of sheep, there was evidence of departure from additivity in respect of field area and time. The implications of these findings for the model are discussed.Significant positive correlations were found between successive time intervals in the number of faecal deposits present in individual subplots of the majority of fields. It is suggested that these arise from the tendency of sheep to use the same portion of a field for resting and rumination when heavy concentrations of faeces are deposited. The effects of this and other possible patterns of grazing and faecal deposition are discussed in relation to the intake of strongyloid infective larvae.The average number of discrete deposits of faeces on pasture produced per sheep per day was greater in flocks producing faeces in pellet form than in flocks producing soft faecal masses. However, within faeces types, there was evidence that the expected number of deposits produced per sheep per day could reasonably be assumed constant.The greater part of this work was carried out while one of us (A.D.D.) was the holder of a C.S.I.R.O. Overseas Research Studentship. For his advice and constructive criticism I am much indebted to Dr H. D. Crofton, under whose supervision this study was undertaken. My thanks go also to Professor J. E. Harris, C.B.E., F.R.S., for the provision of facilities in the Department of Zoology, and to Professor T. K. Ewer and Mr M. R. McCrea for their willing co-operation in the provision and handling of pastures and sheep at the School of Veterinary Science, Langford.Mr David Culpin and Mrs Jean Williams, of the Division of Mathematical Statistics, were responsible for programming and organizing the calculations on the CDC 3200 Computer, and were consistently helpful in proposing and criticizing tests and procedures.